Polarizer devices and methods for making the same

ABSTRACT

A light polarizer includes a plurality of discrete layers of birefringent film disposed in superposed relation on a transparent substrate. The planar components of the optic axes of each successive one of the discrete layers are angularly offset relative each preceding one of the discrete layers by a predetermined angle in one or either a positive or negative direction to provide the light polarizer with selectivity for either right-handed circularly polarized (RHCP) or left-handed circularly polarized (LHCP) incident light. In another aspect of the invention, the layers are of quarter-wave thickness having planar components orthogonally offset to provide selectivity for either right or left linearly polarized light. In a still further aspect of the invention, an optically isotropic layer of quarter-wave thickness is disposed between adjacent birefringent quarter-wave plates.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to optical polarization devices, and moreparticularly to non-absorbing, normal-incidence, reflective polarizingoptical elements employing a multilayer stack of optically birefringentthin film layers obtainable by oblique vapor deposition in vacuum.

2. Background Information

The prior art includes several well-known light polarizing components.For example, prism polarizers, commonly known as Nicol and Glanpolarizers, are based on double refraction of birefringentcrystals--usually calcite. These components utilizepolarization-selective total internal reflection at the interfaces oftwo matched prisms made of birefringent crystals. With these polarizers,one of the linearly polarized light components of incident light isremoved by total internal reflection, while the orthogonal component istransmitted. These prism polarizers offer an extinction ratio of higherthan 100,000 and can cover a relatively wide spectral range. Forexample, calcite polarizers nominally have a useful spectral range of0.35-2.3 μm. However, these crystal polarizers are limited in size bythe scarcity of calcite of good optical quality. In addition, they tendto be expensive, bulky and extremely sensitive to angle of incidence.

A variation of the Glan type polarizer is the cube polarizer disclosedin U.S. Pat. No. 2,403,731 issued to MacNeille. This polarizer consistsof two glass prisms cemented together along their hypotenuses. Thediagonal face of one of the prisms is coated with a multilayerdielectric film stack designed to reflect one linear polarizationcomponent and transmit the orthogonal component. Although much lesscostly than crystalline prism polarizers, MacNeille cubes typicallyexhibit a lower extinction ratio of about 1,000 and can operate over awavelength band of only about 300 nm.

Additional significant prior art includes dichroic polarizers, such asthose commonly referred to as "Polaroid®" sheet polarizers. Thesepolarizers are based on polarization-selective absorption, or dichroism,which was first discovered by Biot circa 1815. Modern sheet polarizersare based on an invention by Edwin Land circa 1928. This technology isreviewed by the inventor in Journal of Optical Society America, 41,957(1951). These widely used sheet polarizers can typically provide anextinction ratio of 1,000 and are of low cost. High extinction ratio ispossible but is achieved at expense of transmittance efficiency. Becausethey are constructed of organic polymers, they possess severe inherentlimitations, i.e. they are limited to low optical power handlingcapability, limited to use at low operating temperature and restrictedto use in the visible spectral range.

Another example of a dichroic polarizer recently introduced by CorningGlass Works of Corning N.Y., is sold under the trademark Polarcor™. Thispolarizer is made of glass which contains elongated submicroscopicsilver particles aligned in the same direction. The silver particlespreferentially absorb light polarized along the long axis of the silverparticles while transmitting light of the orthogonal polarization. Theseglass-based filters are durable and may typically withstand atemperature up to 400° C.; however, they exhibit relatively narrowbandwidth and are limited in operating wavelength range fromapproximately 0.633 to 2.1 μm.

A further example of a dichroic filter includes a metal film obliquelydeposited on a glass substrate as disclosed in U.S. Pat. No. 5,122,907(hereinafter, the '907 patent) issued to Slocum. This dichroic film ismade up of microscopic prolate metal spheroids all aligned in the samedirection on the glass surface. A polarizer of this general type,commercially available from Melles Groit Company of Irvine, Calif., forexample, operates in the near-infrared spectral range and has relativelylow transmittance of only about 20%.

Other important prior art includes polarizers based on organiccholesteric liquid crystalline (CLC) materials. This art has beentaught, for example, by Lee and Jacobs, in a paper published in Journalof Applied Physics, Vol. 68, No. 274 (1990). Cholesteric liquidcrystalline thin films, with molecular alignment in the so-called planartexture, exhibit wavelength- and circular polarization-selectivereflectance over a spectral range which is material dependent. Suchfilms may operate satisfactorily as circular polarizers in manyapplications, but because they are organic materials, they absorbstrongly in the UV spectral range, limiting their operating range.Moreover, their operating temperature is limited to about 140° C. Thesefilms may be used as a transmissive linear polarizers in conjunctionwith a quarter-wave plate which converts the polarization of transmittedlight from circular to linear.

Recently, Iwatsuka et al disclosed polarizing devices based ondiffraction grating based on obliquely deposited thin film in U.S. Pat.No. 5,245,471. One of the limitations of such devices is that the 0-thdiffracted light beam remains unpolarized.

Thus, a need exists for an improved polarizing optics method andapparatus for polarizing light in a manner which achieves a substantialimprovement in efficiency and power handling capability.

SUMMARY OF THE INVENTION

According to an embodiment of this invention, a light polarizer includesa substrate and a plurality of discrete layers of birefringent filmdisposed on the substrate in superposed relation to one another. Eachone of the discrete layers is substantially planar and defines a planardirection. Each one of the discrete layers has an optic axis and aplanar component thereof extending in the planar direction. The planarcomponent of at least one of the plurality of discrete layers isangularly offset from the planar component of an other of the pluralityof discrete layers.

As an enhancement, the planar component of each successive one of thediscrete layers is angularly offset relative each preceding one of thediscrete layers by an oblique, acute angle in one of either a positiveor negative direction to provide the light polarizer with either aright-handed chirality or a left-handed chirality for circularlypolarizing incident light.

The present invention provides, in a second aspect, a light polarizerincluding a substrate and a plurality of birefringent layers superposedover one another on the substrate. Each one of the birefringent layershas a quarter-wave thickness t=λ₀ /4, where λ₀ is a characteristicwavelength of the light polarizer. Each one of the birefringent layersis substantially planar, defining a planar direction, and has an opticaxis and a planar component thereof extending in the planar direction.An optically isotropic layer of quarter-wave thickness is locatedbetween adjacent birefringent layers.

As an enhancement, at least one of the birefringent layers is acomposite layer of superimposed sublayers. Each one of the superimposedsublayers has a sublayar planar component of a sublayer optic axis. Thesublayer planar components of each one of the superimposed sublayersextend substantially parallel to one another in the planar direction.

A still further aspect of the present invention includes a method offabricating a polarizer including the steps of providing a substrate anddepositing a plurality of layers of birefringent film on the substratein superposed relation to one another, each one of the plurality ofdiscrete layers being substantially planar and defining a planardirection. Each one of the plurality of layers are deposited from anoblique deposition angle relative the planar direction, to provide eachone of the layers with an optic axis having a planar component thereofextending in the planar direction. The deposition angle of at least oneof the layers is angularly offset in the planar direction from thedeposition angle of an other of the layers to angularly offset theplanar component of at least one of the layers from the planar componentof an other of the layers.

In yet another aspect of the present invention, a method of fabricatinga light polarizer includes the steps of providing a substrate anddepositing a plurality of birefringent layers superposed over oneanother on the substrate, each one of the plurality of birefringentlayers being substantially planar and defining a planar direction. Eachone of the birefringent layers is provided with a quarter-wave thicknesst=λ₀ /4, where λ₀ is a characteristic wavelength of the light polarizer.Each birefringent layer is deposited from an oblique deposition anglerelative the planar direction, so that each birefringent layer isprovided with an optic axis having a planar component thereof extendingin the planar direction. An optically isotropic layer of quarter-wavethickness is deposited between adjacent birefringent layers.

The present invention thus advantageously utilizes birefringent thinfilm stacks to nominally completely reflect, rather than absorb,normally-incident light polarized in one orientation to provide apolarizer capable of operating at relatively high light intensities andtemperatures.

The above and other features and advantages of this invention will bemore readily apparent from a reading of the following detaileddescription of various aspects of the invention taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic plan view of a polarizer array incorporating aplurality of circular polarizers of the present invention;

FIG. 1B is a schematic plan view of a polarizer array incorporating aplurality of linear polarizers of the present invention;

FIG. 2A is a schematic perspective view of a representative portion of apreferred embodiment of a linear polarizer in accordance with thepresent invention;

FIG. 2B is a graphical representation of the refractive indices of thelinear polarizer of FIG. 2A, for x- and y-polarized light;

FIG. 3A is a view similar to that of FIG. 2A, of another preferredembodiment of a linear polarizer in accordance with the presentinvention;

FIG. 3B is a view similar to that of FIG. 2B, for the linear polarizerof FIG. 3A;

FIG. 4A is a graphical representation of a transmission spectrum forlight polarized in the plane containing the optic axes of thebirefringent layers shown in FIGS. 2A and 3A;

FIG. 4B is a graphical representation of a transmission spectrum forlight polarized in a plane perpendicular to the plane containing theoptic axes of the birefringent layers shown in FIGS. 2A and 3A;

FIG. 5A is a view similar to that of FIG. 2A, of another preferredembodiment in accordance with the present invention;

FIG. 5B is a view similar to that of FIGS. 2B and 3B, for the polarizerof FIG. 5A;

FIG. 6A is a view similar to that of FIG. 2A, of another preferredembodiment in accordance with the present invention;

FIG. 6B is a view similar to that of FIGS. 2B, 3B and 5B, for linearpolarizer shown in FIG. 6A;

FIG. 7 is a graphical representation of transmittance spectra for lightpolarized parallel and perpendicular to the optic axes of thebirefringent thin film stacks of FIGS. 5A and 6A;

FIG. 8A is an exploded, schematic perspective view of a representativeportion of a preferred embodiment of a circular polarizer of constantpitch distribution, in accordance with the present invention;

FIG. 8A1 is a series of schematic plan and side elevational views oflayers of a circular polarizer portion of the type shown in FIG. 8A;

FIG. 8A2 is a graphical representation of angular offset and layerthickness for the layers of the circular polarizer portion shown in FIG.8A1;

FIG. 8B is a view similar to that of FIG. 8A, of a representativeportion of a preferred embodiment of a circular polarizer of gradientpitch distribution, in accordance with the present invention;

FIG. 8B1 is a series of schematic plan and side elevational views oflayers of a circular polarizer portion of the type shown in FIG 8B;

FIG. 8B2 is a graphical representation of angular offset and layerthickness for the layers of the circular polarizer portion shown in FIG.8B1;

FIG. 8C is a view similar to that of FIG. 8A, of a representativeportion of a further embodiment of a circular polarizer of gradientpitch distribution, in accordance with the present invention;

FIG. 8C1 is a series of schematic plan and side elevational views oflayers of a circular polarizer portion of the type shown in FIG. 8C;

FIG. 8C2 is a graphical representation of angular offset and layerthickness for the layers of the circular polarizer portion shown in FIG.8C1;

FIG. 9A is a graphical representation of reflectance spectra forright-handed circularly polarized and left-handed circularly polarizedlight incident on the circular polarizer of FIG. 8A;

FIG. 9B is a graphical representation of the pitch distribution of acircular polarizer of the type shown in FIG. 8A;

FIG. 10 is a graphical representation of a reflectance spectrum of acircular polarizer of the type shown in FIG. 8A;

FIG. 11A is a graphical representation of reflectance spectra ofcircular polarizers of the type shown in FIG. 8, having a non-constantpitch distribution;

FIG. 11B is a graphical representation of the pitch distribution of acircular polarizer of the type shown in FIG. 11A;

FIG. 12A is a schematic representation of a birefringent thin filmduring a step in the fabrication thereof;

FIG. 12B is a graphical representation of levels of birefringenceobtained experimentally by fabricating SiO films at different depositionangles in the manner shown in FIG. 12A;

FIG. 13 is a schematic representation of a system suitable forfabricating the thin film as shown in FIG. 12A;

FIG. 14 is a schematic representation of a high throughput systemsuitable for fabricating the thin film as shown in FIG. 12A;

FIG. 15 is a schematic representation of a system suitable forcontinuous fabrication of the thin film as shown in FIG. 12A, on aflexible substrate;

FIG. 16 is a schematic drawing of another system suitable for continuousfabrication of a thin film as shown in FIG. 12A, on flexible substrate;

FIG. 17A is a diagrammatic representation of a polarizing device of thepresent invention, in a reflective mode, including light incidentthereon;

FIG. 17B is a diagrammatic representation of the polarizing device ofFIG. 17A, in a transmittive mode, including light incident thereon;

FIG. 18A is a diagrammatic side elevational representation of apolarizer of the type shown in FIG. 8A, with some layers thereof notshown, and including light incident thereon;

FIG. 18B is a diagrammatic side elevational representation of apolarizer of the type shown in FIG. 8B, with some layers thereof notshown, and including light incident thereon;

FIG. 19A is an elevational schematic representation of a representativeportion of an additional embodiment of a polarizer of the presentinvention;

FIG. 19B is a graphical representation of the pitch distribution of thepolarizer portion of FIG. 19A;

FIG. 20A is an elevational schematic representation of a representativeportion of a further embodiment of a polarizer of the present invention;

FIG. 20B is a graphical representation of the pitch distribution of thepolarizer portion of FIG. 20A;

FIG. 21 is an elevational schematic representation of a still furtherembodiment of a polarizer of the present invention;

FIG. 22A is a graphical representation of the refractive indices of alinear polarizer of the type shown in FIGS. 2A and 3A, for x-polarizedlight;

FIG. 22B is a view similar to that of FIG. 22A, for an alternateembodiment of a linear polarizer; and

FIG. 22C is a view similar to that of FIGS. 22A and 22B of a stillfurther embodiment of a linear polarizer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For clarity of exposition, like features shown in the accompanyingDrawings shall be indicated with like reference numerals and similarfeatures such as shown in alternate embodiments shall be indicated withsimilar reference numerals.

Briefly described, the present invention includes birefringent thin filmstacks that utilize properties of optical interference to reflect,rather than absorb, normally incident light of a particular polarizationorientation. As shown, a light polarizer 426 (FIG. 8A) includes aplurality of discrete layers 438 of birefringent film disposed insuperposed relation on a substrate. The planar components c' of theoptic axes c of each successive one of the discrete layers are angularlyoffset relative each preceding one of the discrete layers by apredetermined angle β in one of either a positive or negative directionto provide the light polarizer with selectivity for either right-handedcircularly polarized (RHCP) or left-handed circularly polarized (LHCP)incident light. In another embodiment of the invention, a polarizer 226(FIG. 5A) includes birefringent layers 230 and 32 of quarter-wavethickness, each having optic axes c orthogonally offset in the planardirection to provide selectivity for either of two orthogonal linearlypolarized light components. For convenience, the two orthogonalcomponents are chosen along the x- and y-axes as shown herein and arereferred to hereinafter as x-polarized and y-polarized light,respectively. In a still further aspect of the invention, a polarizer 26(FIG. 2A) includes an optically isotropic layer 34 of quarter-wavethickness disposed between adjacent birefringent quarter-wave layers.

Where used in this disclosure, the term `polarizer` shall refer to anystack of superposed birefringent layers, as taught herein, capable ofexhibiting polarization-selective and/or wavelength-selectivereflectance, regardless of the number of layers thereof. The term`thickness` when used herein in reference to a layer or any portionthereof, shall refer to the optical thickness, as defined by physicalthickness divided by the refractive index thereof. The term `isotropic`shall mean that the refractive index of a material is independent of thepolarization direction of light. Similarly, the term `anisotropic` shallmean that the refractive index of a material is polarizationdirection-dependent. The terms `homogeneous` and `inhomogeneous` meanthat the refractive index of a particular layer is, respectively,position-independent and position-dependent. For convenience,inhomogeneous layers are assumed to be stratified, meaning that theoptical properties thereof are constant throughout each planeperpendicular to a fixed direction defined by the z-axis of theCartesian coordinate system 21 shown in the drawings. Thus, therefractive index n of an inhomogeneous layer has a functionaldependence, n(z), which may depend on the polarization direction oflight traveling therethrough. The refractive index for these twocomponents is different. Any reference in this disclosure to an x-, y-or z-axis or direction shall refer to directions relative to Cartesiancoordinate system 21 as shown in the Drawings. Moreover, the terms`planar direction` and `planar component` shall refer to a direction,for example, of an optic axis, in the xy-plane of the Cartesiancoordinate system 21.

Referring to FIG. 1A, a circular polarizer array 20 may be fabricatedutilizing circular polarizers 22 and 22' of the present inventionarranged in a crossed or alternating pattern. Referring to FIG. 1B, alinear polarizer array 24 may be similarly fabricated utilizing crossedlinear polarizers 26 and 26' of the present invention. As shown,polarizers 22' and 26' are similar, but orthogonally polarized examplesof polarizers 22 and 26 respectively. The term `orthogonal` when usedherein to describe polarization directions, shall be used in itstraditional vector geometrical sense, in which `orthogonal vectors` arethose whose dot products are zero. As such, orthogonal vectors may beeither a pair of perpendicular straight vectors, or alternatively, apair of complex vectors, such as those representing circularpolarization. Accordingly, for simplicity, throughout this disclosure,polarizers of only one articular orientation may be described with theunderstanding that such descriptions will also apply to polarizers oforthogonal orientations.

As shown in FIG. 2A, linear polarizer 26 includes a stack of thin filmlayers 30, 32, . . . , 34 and 36 on a substrate 38. The substrate iseither optically isotropic or anisotropic. Each layer 30, 32, . . . , 34and 36 is alternately optically isotropic and anisotropic. Each layermay be either homogeneous or inhomogeneous. For light normally incident,that is, traveling in the -z-direction, e.g. from the air, polarizer 26has two effective refractive indices, n_(x) (z) and n_(y) (z), for lightpolarized along x- and y-axes, respectively.

Those skilled in the art will recognize that light normally incident ona film stack such as polarizer 26 may effectively include two orthogonalpolarization components, one extending along the x-axis and the otheralong y-axis. Thus, optical reflectance or transmittance of the incidentlight can be analyzed with respect to these components. Opticalreflectance and transmittance properties of a multilayer stack of filmsare well known in the art. Typical multilayer coatings are either usedto enhance reflectance, a so-called high-reflector coating, or to reducereflectance, a so-called anti-reflection coating.

Turning back to FIG. 2A, the reflective polarizing thin film coatingconsists of a plurality of preferably homogeneous layers that arealternately isotropic and birefringent. As shown, layers 30 and 32 forma pair of isotropic and birefringent layers, respectively, (Pair 1) andlayers 34 and 36 form another similar pair, (Pair M). For simplicity ofillustration, layers 30 and 34 are optically isotropic, fabricated fromthe same material referred herein as material B (which will be discussedin greater detail hereinafter) and having a refractive index n_(B).However, layers 30 and 34 may be fabricated from nominally any materialproviding dissimilar indices of refraction to layers 32 and 36, as willbecome apparent hereinafter. Layers 32 and 36 are opticallybirefringent, each having an optic axis c in the xz-plane, disposed at apredetermined angle γ to the incident direction or z-axis. Thus, asshown, a planar component c' of each optic axis c is also in thexz-plane. Formation of optic axis c and values for angle γ for theembodiments of the present invention will be discussed hereinafter withrespect to FIG. 12A. Layers 32 and 36 are preferably fabricated from thesame material A (also to be discussed hereinafter) and have refractiveindices denoted by n_(Ax) and n_(Ay) for light polarized in the x- andy-directions, respectively. Typically, n_(Ax) >n_(Ay) as will becomeclear hereinafter to those skilled in the art. Layers 30, 32, . . . , 34and 36 are of quarter-wave thickness, that is, n_(Ax) t_(A) =n_(B) t_(B)=λ₀ /4, where t denotes the layer thickness, subscripts A and B indicatethe layer material and λ₀ is the characteristic wavelength or centerwavelength of the reflectance or transmittance band of the polarizer. Inthis regard, reflectance and transmittance (assuming negligible losses)are generally equal but opposite measurable characteristics of apolarizer. The relation between the reflectance R(λ) and transmittanceT(λ) is given by

    R(λ)+T(λ)=100%

Thus, the terms reflectance and transmittance, when utilized herein inconjunction with a polarizer of the present invention, areinterchangeable. Moreover, where succeeding pairs of layers, such aspair M, are substantially identical to a first pair, such as pair 1,throughout the remainder of this disclosure, only one pair may bediscussed with the understanding that such discussion will berepresentative of all similar pairs of layers.

Optically isotropic film layer 30 is preferably deposited on substrate38 at normal incidence to the substrate in a conventional manner.Alternatively, the isotropic layer may be obtained by deposition at anoblique angle with the substrate rotating, as in a manner to bediscussed hereinafter, so that the resulting film is opticallyisotropic. The thickness t_(B) of layer 30 is such that n_(B) t_(B) =λ₀/4. In contrast, optically birefringent film layer 32 is deposited ontothe layer 30 at a deposition angle θ (FIG. 12A) predetermined to providelayer 30 with optic axis c oriented at predetermined angle γ asmentioned hereinabove and as will be discussed hereinafter with respectto FIG. 12A.

Film layer 32 thus exhibits an optical birefringence Δn_(A), whereinΔn_(A) =n_(Ax) -n_(Ay) for light incident normal to the substrate, wheren_(Ax) is the index of refraction for x-polarized light and n_(Ay) isthe index of refraction for y-polarized light incident on layer 32. Thethickness t_(A) of layer 32 is such that n_(Ax) t_(A) =λ₀ /4. To promoteefficiency of polarizer 26, the refractive index in the y direction,n_(Ay), of layer 32 closely matches the refractive index of layer 30,i.e., n_(Ay) =n_(B). Thus, using terminology common in the art, n_(Ax)=n_(H) and n_(Ay) =n_(L). This deposition procedure is then repeateduntil a predetermined number, M, of birefringent-isotropic pairs havebeen disposed in a stacked or superimposed manner onto substrate 38. Theresulting structure exhibits strong reflection over a spectral bandcentered at wavelength λ₀ for x-polarized light and exhibits hightransmittance for y-polarized light. Polarizer 26 thus rejects lightpolarized in one plane of polarization by reflection, rather than byabsorption. This aspect enables the polarizer of the present inventionto be inherently capable of withstanding higher incident intensitiesthan conventional dichroic polarizers.

Referring now to FIG. 2B, for light polarized along the x-axis, therefractive index profile is a periodic function in z: n_(air) (n_(H)n_(L))^(M) n_(substrate), where n_(H) =n_(Ax), n_(L) =n_(B) =n_(Ay) andM is the number of pairs 1. The refractive indices of air and substrateare denoted by, respectively, n_(air) and n_(substrate).

The polarizing performance of a thin film stack can be calculated usingtransfer matrix or characteristic matrix techniques commonly used inmultilayer film theory, for example, given in E. Hecht and A. Zajac,Optics, Addison-Wesley Publishing Co., Reading, Mass., 1974 (pages311-316) the cited pages of which are hereby incorporated by referencein their entireties, herein.

A polarizer of the present invention, such as polarizer 26, may begenerally characterized as a type a(HL)^(M) s structure, where s denotesa substrate, a air and HL for alternating quarter-wave layers with highand low refractive indices, respectively. The number of HL layers pairsis M. Where incident light is normal to the substrate surface, a maximumreflectance at λ₀ for light polarized in the xz-plane is: ##EQU1## andthe maximum reflectance for light polarized in the yz-plane is: ##EQU2##

Attention is thus directed to FIGS. 4A and 4B which show transmittancespectra for a representative example of a multilayer polarizer 26fabricated as discussed hereinabove. In this example, birefringentlayers 32 and 36 have refractive indices of n_(H) =1.72 and n_(L) =1.66,the isotropic layers 30 and 34 have a refractive index n_(L) =1.66 andM=150. The substrate material is conventional soda lime glass (n_(s)=1.52) and the designed characteristic wavelength or center wavelengthλ₀ of the reflectance band is 550 nanometers, λ₀ =550 nm. Incident lightwas normal to the substrate surface. Maximum reflectances in the xz- andyz-planes calculated using formulas (1a) and (1b) hereinabove yield anextinction ratio, defined as transmittance ratio of x-polarized toy-polarized light, of nearly 10,000. This ratio is better than thattypically achieved using MacNeille or dichroic polarizers. The bandpassspectral width is approximately 10 nanometers.

It is expected that a larger birefringence Δn for the birefringentA-layers will result in a larger polarizing bandwidth. In this regard,the reflectance bandwidth is given by the following equation (1c) thatappears in subsection 43, Section 8 Filters and Coatings Based onInterferences in Thin Films by J. A. Dobrowolski, in Handbook of Optics,W. G. Driscoll and W. Vaughan, ed., McGraw-Hill Book Company, New York1978, (hereinafter Filters and Coatings) which subsection is herebyincorporated by reference in its entirety herein. The reflectancebandwidth is given by:

    Δλ/λ=(4/π)sin.sup.-1 [(1-n.sub.H /n.sub.L)/(1+n.sub.H /n.sub.L)].                          (1c)

Larger n_(H) /n_(L) (or birefringence Δn=n_(H) -n_(L)) results in alarger Δλ/λ (or bandwidth Δλ centered at λ). This is a consequence ofoptical interferences from a periodic structure as shown for x-polarizedlight in FIG. 2B.

Thus, as shown by the above example, the use of a multilayer coatinghaving alternating birefringent and isotropic layers as taught by thesubject invention effectively utilizes the optical characteristics ofbirefringent layers. For the x-polarized light, polarizer 26 is ahigh-reflector, utilizing the optical interference effects of thestacked layers. For y-polarized light, the film stack is isotropic andthus, a high-transmitter. This reflectance difference for the twoorthogonally polarized components is utilized as a polarizing element.While this example employs 150 thin-film pairs, this number may besignificantly reduced by utilizing materials with higher birefringence.Polarizers fabricated with such layers of relatively higherbirefringence will have transmittance and reflectance bands of greaterbandwidth, for the reasons discussed hereinabove with respect toequation (1c).

Relatively higher birefringence also tends to provide higher extinctionratios as shown by the example described in conjunction with equation(1a), hereinabove, In this regard, referring back to the examplediscussed hereinabove in conjunction with FIGS. 4A and 4B, for a highestextinction ratio, R_(x-polarized) light should approach unity. If n_(Ax)is much larger than n_(Ay) (i.e., larger birefringence), the term(n_(Ay) /n_(Ax))^(2M) is smaller for a given M (number of pairedlayers), wherein R_(x-polarized) light is closer to unity. Numericalexamples are shown in the following table:

    ______________________________________                                        (n.sub.Av /n.sub.Ax).sup.2M                                                                  (n.sub.Av /n.sub.Ax).sup.2M                                                                (n.sub.Av /n.sub.Ax).sup.2M                       ______________________________________                                        M = 5 (0.9).sup.10 = 0.34868                                                                     (0.8).sup.10 = 0.10737                                                                     (0.7).sup.10 = 0.02825                        10    (0.9).sup.20 = 0.12158                                                                     (0.8).sup.20 = 0.01153                                                                     (0.7).sup.20 = 0.00080                        15    (0.9).sup.30 = 0.04239                                                                     (0.8).sup.30 = 0.00124                                                                     (0.7).sup.30 = 0.00002                        ______________________________________                                    

Turning now to FIG. 3A, an alternate embodiment of the present inventionis shown as polarizer 126. In this embodiment, optically birefringentlayers 132 and 136 are composite layers, including a plurality ofbirefringent sublayers having optic axes c (and planar components c') inthe xz-plane. Layer 132 includes sublayers 134 and 135, while layer 136includes sublayers 138 and 140. Individually, each sublayer preferablyhas substantially the same degree of birefringence as birefringentlayers 32 and 36 (FIG. 2A), but their optic axes c of adjacent sublayersare preferably each disposed at alternating angles of +γ and -γ in thexz-plane, as shown with respect to layers 134 and 135. These sublayersare obtained by thin film deposition of material A alternately atoblique angles +θ and -θ, (FIG. 12A) respectively, in the xz plane. Thetotal thickness of each composite layer 132, including sublayers 134 and135, is a quarter wave, given by t₁₃₄ +t₁₃₅ =λ₀ /4n_(H) where t denotesthe thickness of a sublayer. Similarly, the combined thickness ofsublayers 138 and 140 is also a quarter wave, t₁₃₈ +t₁₄₀ =λ₀ /4n_(H).Thus, optically, composite layer 132 is equivalent to a singlebirefringent layer of quarter wave thickness with refractive indicesn_(Ax) and n_(Ay). In this regard, as shown in FIG. 3B, the refractiveindices of linear polarizer 126, for x- and y-polarized light, aresubstantially identical to those shown in FIG. 2B for polarizer 26.However, it may be advantageous to deposit a film as several relativelythin sublayers, rather than fewer thicker layers, to, for example, limitfractal columnar growth and facilitate film uniformity.

In this regard, one skilled in the art will recognize that as filmthickness increases, columnar growth therein becomes increasinglyfractal-like, leading to excess light scattering of the film. As taughtherein, this problem may be avoided or minimized by limiting such growththrough use of relatively thin film layers. In this regard, each timedeposition of one layer is stopped, an another layer begun, such as bychanging deposition angle θ, the columnar growth is stopped and newgrowth starts again from a `zero` datum. In light of the above, althoughit is preferred to deposit each adjacent sublayer at alternate obliqueangles +θ and -θ, it is contemplated that the present invention may bepracticed by depositing adjacent discrete sublayers at the same angle θ.

Another advantage, however, of depositing sublayers at alternate obliqueangles +θ and -θ is that such an approach enables the sublayers tocompensate one another to provide the composite layer with improveduniformity in thickness. Inherent to the oblique deposition geometryshown in FIG. 13, the film tends to be non-uniform depending onproximity to an evaporant source 566. When deposited at +θ, the layer atthe top region of the substrate (farther away from the source) will bethinner than the film at the bottom region (closer to the source). Whendepositing at -θ, the substrate is effectively rotated 180 degrees aboutthe z-axis. The film deposited at -θ thus has similar, but oppositenon-uniformity in thickness relative the +θ film. The non-uniformitiesof these two sublayers, i.e. sublayers 134 and 135, thus compensate oneanother such that the thickness of the resulting composite film layer(i.e. layer 132) is substantially uniform.

Referring now to FIG. 5A, in another exemplary embodiment, a linearpolarizer 226 includes pairs of alternating birefringent layers 230 and32, and 234 and 36 disposed on substrate 38. Layers 230 and 234 areobliquely deposited in the yz-plane with material B. The optic axisc_(B), and the planar component thereof, c_(B) ', of each layer 230 and234 is in the yz-plane with n_(y) (B)>n_(x) (B). These layers are each aquarter wavelength thick (λ₀ /4n_(L)), where λ₀ is the characteristicwavelength of the linear polarizer. Layers 32 and 36 are obliquelydeposited with material A in the xz-plane as discussed hereinabove, withoptic axis C_(A) of these layers in the xz-plane, with n_(x) (A)>n_(y)(A). The materials and deposition process are chosen such that n_(y)(A)=n_(y) (B). Accordingly, to utilize terminology common in the art,n_(x) (B) is defined as n_(L) (n_(x) (B)=n_(L)) and n_(x) (A) is definedas n_(H) (n_(x) (A)=n_(H)). This deposition procedure is then repeateduntil a stack of M" paired birefringent layers have been applied tosubstrate 38. As shown, layers 234 and 36 form the last pair of thestack, Pair M".

As shown in FIG. 5B, the profile of refractive indices for lightpolarized in the x- and y-axes is similar to those shown in FIGS. 2B and3B. Polarizer 226 exhibits strong reflection over a band centered at thewavelength λ₀ for light polarized in the x-direction as the multilayerstack consists of layers with different refractive indices n_(H) andn_(L). Polarizer 226 exhibits high transmittance for the light polarizedin the y-direction as the multilayer stack is effectively an isotropicmedium, n_(y) (A)=n_(y) (B). Thus, polarizer 226 functions in the mannerdescribed hereinabove with regard to polarizers 26 and 126, toselectively reflect light of one linear polarization component andtransmit the orthogonal component, to thus function as a linearpolarizer. The advantage of polarizer 226 over the embodiment ofpolarizer 126 (FIG. 2A) is that be effectively adding the birefringencesof two layers as shown and described, Δn (in the x-direction), for everypair of quarter wavelength layers, is relatively larger. This aspectenables polarizer 226 to be advantageously fabricated with relativelyfewer quarter wavelength pairs, for lower relative complexity andmanufacturing expense. This larger birefringence may also providerelatively increased bandwidth as discussed hereinabove.

Referring now to FIG. 7, transmittance spectra 280 and 282 for the x-and y-polarized light, respectively, is provided for an illustrativeexample of a polarizer 226 having 27 quarter wavelength pairs, effectiveΔn=0.16 and an average refractive index of 1.8. As shown, this exampleexhibits a relatively wide bandwidth (approximately 20 to 30 nm) and ahigh extinction ratio (approximately 100). Moreover, it is expected thatthe extinction for this example may be improved by increasing R_(x)^(max), as set forth hereinabove, with a larger number of pairs anddecreasing R_(y) ^(max), as also set forth hereinabove, by choosingn_(substrate) =n_(B) =n_(y) (A).

Turning now to FIG. 6A, an alternate embodiment of linear polarizer 226(FIG. 5A) is shown as polarizer 326 and includes optically birefringentcomposite layers 330, 332, 334 and 336. Each composite layer 330, 332,334 and 336 includes a plurality of birefringent sublayers.

As shown, layers 330 and 334 each include sublayers 338 and 340.Sublayers 338 and 340 each preferably have the same birefringence as oneanother, but with their optic axes c_(B) (and planar components, notshown) alternately disposed at an angle of +γ and -γ, respectively, inthe yz-plane. These sublayers are obtained by oblique thin filmdeposition of material B in the yz plane alternately at oblique angles+θ and -θ, respectively, relative the z-axis. The combined thickness ofsublayer 338 plus sublayer 340 is a quarter wave, given by t₃₃₈ +t₃₄₀=λ₀ /4n_(L) where t denotes the thickness of the respective sublayer.Layers 330 and 334 are optically equivalent to quarter wavelengthbirefringent layers 230 and 234 described hereinabove, having refractiveindices n_(Bx) and n_(By).

Similarly, birefringent layers 332 and 336 each include sublayers 342and 344. Sublayers 342 and 344 each have substantially identicalbirefringence, but with their optic axes alternating at an angle +γ and-γ, respectively, in the xz-plane. These sublayers are obtained byoblique thin film deposition of material A in the xz plane alternatelyat oblique angles +θ and -θ, respectively. The total thickness of eachpair of sublayers 342 and 344 is a quarter wave, t₃₄₂ +t₃₄₄ =λ₀ /4n_(H)where t denotes the thickness of sublayers. The optical reflectance ortransmittance of polarizer 326 is similar to that of polarizer 226 (FIG.5A), as both have similar profiles of refractive indices as shown inFIG. 6B. However, it may be advantageous to deposit composite sublayersto achieve film uniformity as discussed hereinabove.

There are several other configurations suitable for creating thin filmmultilayer linear polarizing coatings of the present invention. Notably,conventional optical techniques such as utilization of a disordered filmstack, i.e. a film stack of alternating layers not of exactly quarterwavelength thickness, as well as conventional Fourier transform methods,may be utilized in a manner familiar to those skilled in the art, incombination with the teachings of the present invention, to designpolarizers of substantially any desired reflectance or transmittancespectra. An example of such a conventional disordered film stacktechnique utilized to produce an optical reflector is disclosed by Zhanget al in Applied Physics Letters, 67, 679(1995), while a common Fouriertransform technique is described by Dobrowolski and Lowe, in AppliedOptics, 17, 3039 (1978), both of which are hereby incorporated byreference herein in their entireties.

Notably, the advantage of a disordered stack over a traditionalquarter-wave stack is that a broader reflective spectral band may beachieved. All of the embodiments of the present invention discussedherein may be fabricated using such a disordered stack. In this regard,a reflective coating example utilizing layers of other than quarter-wavethickness is shown in subsection 41 of the above-referenced Filters andCoatings, which subsection is hereby incorporated by reference in itsentirety, herein. Specifically, an example is given for a pair of layersof thickness n_(A) t_(A) +n_(B) t_(B) =λ₀ /2. Other examples ofhigh-reflector coatings of non quarter-wave film stacks are familiar tothose skilled in the coating art.

It should thus be apparent to the skilled artisan that a multilayer filmstack of birefringent layers having substantially any thickness, thatutilizes the birefringence of the layers to reflect one linearlypolarized light component to a substantially greater degree than anorthogonally polarized light component, is within the spirit and scopeof the present invention.

Further embodiments of the present invention comprise circularpolarizers. Referring now to FIG. 8A, a circular polarizer 426 includesa stack of superposed birefringent layers 438. A single helical pitch isshown, with the understanding that additional layers and concomitantly,additional pitches, are preferably superimposed or stacked thereon toimprove performance thereof according to Eq 2d discussed hereinbelow.Each layer 438 is effectively rotated about the z-axis relative toimmediately adjacent layers 438. In this regard, optic axis c of eachlayer 438 has a planar component c' disposed in the xy-plane as shown.The direction of component c' of each layer 438 is rotatably offsetrelative to that of each adjacent layer 438 to a predetermined extentabout the z-axis. The direction of the optical axis c of eachbirefringent layer 438, is given by (β, γ), where β is the angle betweenthe x-axis and the x. component c', and γ is the angle between c andz-axis as defined hereinabove. As shown, angle β is preferably constantfor each layer, β=360°/J, where J is an integer. The optical axis c, asshown by x. component c', of each successive layer 438, is rotated aboutthe z-axis either clockwise or counterclockwise by an equalpredetermined amount, to trace a helix along the z-axis. The pitch orperiod, P₀, is the thickness of the number of layers required tocomplete a full 360 degree revolution of components c'. In the exampleshown, polarizer 426 requires six layers to complete a full evolution,as x. component c' of each layer is oriented at (0°, γ), (60°, γ),(120°, γ), (180°, γ), (240°, γ) and (300°, γ), respectively. A polarizerof this type is defined as right-handed (RH) chiral film is β>0, andleft-handed (LH) chiral film if β<0.

Circular polarizers of the type described hereinabove and shown withrespect to polarizer 426 have a reflectance band for circularlypolarized light of a bandwidth:

    Δλ≈(Δn/n.sub.av)λ.sub.0  (2a)

centered at the characteristic wavelength

    λ.sub.0 =n.sub.av P.sub.0,                          (2b)

where n_(av) is the average index of refraction, given by (n_(x)+n_(y))/2. Variables n_(x) and n_(y) are refractive indices along the x-and y-axes, respectively. Thus, the wavelength-dependent circulardichroism can be controlled by the deposition rate, inclination of thefilm material itself, as will be taught hereinafter. The reflectance (R)vs wavelength is given by

    R=sin h.sup.2 {κL[1-(δ/κ).sup.2 ].sup.1/2 }/cos h.sup.2 {κL[1-(δ/κ).sup.2 ].sup.1/2 }-(δ/κ).sup.2(2c)

where δ=2πn_(av) (1/λ-1/λ₀), δ/κ=2(n_(av) /Δn)(1-λ/λ₀) and L is thethickness of the chiral film stack.

The peak reflectance R_(P) is:

    R.sub.P =tan h.sup.2 [π(Δn/n.sub.av)(L/P)].

The selective reflectance arises from Bragg-type optical interferencesfamiliar to one skilled in the art.

FIG. 9A illustrates the calculated reflectance spectra of a right-handedpolarizer of the type described hereinabove with regard to polarizer426, for right-handed circularly polarized (RHCP) and left-handedcircularly polarized (LHCP) light is shown at 480 and 482, respectively.The design parameters were n_(av) =2.2, Δn=0.2, λ₀ =550 nm, P₀ =250 nm,the film stack has 10 pitches (L/P₀ =10) and each pitch consists of 8layers of same thickness t (t=P₀ /8). The pitch at any given point alongthe z-axis (P(z)) is constant over the entire stack, as illustrated inFIG. 9B. As shown, a pitch P(z) at any particular layer of polarizer426, divided by initial pitch P₀ (the pitch closest to the substrate isequal to one.

Referring now to FIG. 10, calculated and experimental reflectancespectra 484 and 486, respectively, are provided for an additionalexample. In this example, a left-handed circular polarizer as taught bythe present invention was provided, having a stack of SiO film layerscyclically rotated as discussed hereinabove. The pitch of the chiralfilm consisted of eight birefringent layers and was constant. Thelayers, which were all of the same thickness, were deposited at anoblique angle of θ=65°. A total of 104 layers were deposited on a glasssubstrate which was successively rotated counterclockwise by 45° aftereach layer was deposited. The substrate was rotated 13 complete turns,resulting in a chiral film with 13 pitches. The peak reflectance wasabout 35% for the chiral film with opposite chirality, i.e. for RHCPlight. It is anticipated that a polarizer constructed according to thisexample having approximately 54 pitches would have a 100% reflectance ofRHCP light.

By changing the pitch distribution, across the film stack, or providingthe stack with a "gradient pitch distribution", a relatively broaderreflectance band of a circular polarizer may be obtained. A gradientpitch distribution may be provided by increasing the angle β, byincreasing the thickness of succeeding layers, or by a combination ofthese approaches. In this regard, mathematically, a pitch gradient meansthat the derivative dP(z)/dz is non-zero. For a linear gradient pitch,dP(z)/dz=constant, to indicate that the rate of change in pitch isconstant. Since the polarizer is a stack of discrete layers, thederivative may be expressed as a difference ratio ΔP(z)/Δz, asillustrated in Table I hereinbelow, in connection with varying layerthickness. This difference ratio ΔP(z)/Δz may thus be affected by thechanging ΔP(z) and/or Δz.

Thus, for example, referring now to FIG. 8B, a polarizer 426' similar topolarizer 426 (FIG. 8A) may be fabricated with a gradient pitchdistribution by increasing angle β for each succeeding layer, whilekeeping the thickness of each layer 438 equal. For example, β may be35°, 45°, 55°, 65°, 75°, 85°, . . . , for an initial pitch P₀ of 6layers, as shown, with β increasing by 10° for each succeeding layer,respectively. The accumulative angular offsets are35°+45°+55°+65°+75°+85°=360.degree., a complete turn.

Referring now to FIGS. 8B1 and 8B2, the rotation of c' for thesuccessive layers, as well as and side view showing uniform layerthickness, is shown. It should be clear that there are fewer layers perpitch as angle β increases. In this regard, a pitch at any particularpoint along the z-axis may include a non-whole number of layers, i.e.the number of layers required to make a complete 360° rotation for c' isnot an integer. As shown, a complete pitch from layer 7 in the +zdirection is complete between layer 17 and layer 12.

As an alternative, as shown in FIGS. 8C, 8C1 and 8C2, a polarizer 426"is similar to polarizer 426', with the exception that it may be providedwith a gradient pitch distribution by increasing layer thickness forsucceeding layers, while keeping β constant. The following tableillustrates a gradient pitch distribution of polarizer 426" in whichtwist angle β is constant at 60°, so that the pitch is the thickness ofsix layers. Table I below thus shows the instantaneous pitch at anyparticular layer or point along the z-axis, calculated as thickness ofsix layers of the given thickness.

                  TABLE I                                                         ______________________________________                                        Layer    1     2     3   4   5   6   7   8   9   10  11                                                    . . .                                            ______________________________________                                        Thickness, unit                                                                        0.1   0.2   0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0                          Pitch at layer 1                                                                        0.6                                                                 Pitch at layer 2                                                                                1.2                                                         Pitch at layer 3                                                                                       1.8                                                  Pitch at layer 4                                                                                          2.4                                               ______________________________________                                    

As mentioned hereinabove, as opposed to a polarizer 426 (FIG. 8A) ofconstant pitch, alternate embodiments 426' and 426" have a pitch (P(z))that progressively varies as a function of the location along the z-axisthereof. Pitch P(z) at any point along the z-axis of polarizers 426' and426", is given by the following equation, where z equals the distance inthe z-direction from the substrate (not shown) and δP is the totalchange in pitch along the entire length L of the polarizer:

    P(z)=P.sub.0 +(z/L)δP.                               (3)

Turning to FIG. 11A, reflectance spectra of examples of polarizers 426'and 426" are shown with their corresponding pitch distributionsgraphically shown in FIG. 11B. As shown in FIG. 11A, a polarizer 426(FIG. 8A) having constant pitch (δP=0) has a transmittance curve 441. Apolarizer 426' (FIG. 8B) and 426" (FIG. 8C) having a progressivelyincreasing pitch, where δP=0.05P₀, yields transmittance curve 443 ofbroader bandwidth than curve 441. Similarly, further increases in δP, to0.10P₀ and 0.15P₀, yield even broader bandwidth curves 445 and 447,respectively. As shown in FIG. 11B, the pitch distributions of thecircular polarizers exhibiting curves 441, 443, 445 and 447 of FIG. 11Aare shown graphically at plots 449, 451, 453 and 455, respectively. Asshown, these plots are straight lines and thus indicative of lineargradients. The skilled artisan will recognize, however, that a polarizerhaving a nonlinear gradient distribution of P(z), according to Equation3 hereinabove, may also be fabricated. The corresponding pitchdistribution of such a polarizer, would be curved when plotted in amanner similar to that of FIG. 11B.

This broadening of the reflectance bandwidth by varying pitchdistribution may be further explained by reference to FIGS. 18A and 18B.For a polarizer 426 of constant pitch, P₁ as shown in FIG. 18A, thereflectance band is centered at the characteristics wavelength λ₁=n_(av) P₁ with a bandwidth Δλ=(Δn/n_(av))λ₁ where n_(av) is the averageindex of refraction of the material and Δn birefringence. For a filmwith a gradient pitch distribution, (P₁ to P₂), the reflectance band canbe approximately n_(av) (P₂ -P₁). With reference to FIG. 18B below, thefilm region with its pitch P₁ reflects light centered at the wavelengthλ₁ =n_(av) P₁ and the film region with its pitch P₂ reflects lightcentered at λ₂ =n_(av) P₂ with a bandwidth. As the pitch of film iscontinuously varied from P₁ to P₂, a continuous reflectance band from λ₁to λ₂ is reflected from such a film, thereby providing a broaderbandwidth than simply the λ₁ bandwidth of the constant pitch example ofFIG. 18A.

Although the foregoing description discusses broadening the polarizingband of circular polarizers by varying the pitch along the z-axis,similar broadening may be obtained for linear polarizers. Referring nowto FIGS. 22A, 22B and 22C, the polarizing bands of linear polarizers(e.g. polarizers 26 and 126 of FIGS. 2A and 3A) may be broadened bychanging the quarter-wave thickness of a layer-pair. Referring now toFIG. 22A, the refractive indices of a linear polarizer 26 or 126 areindicative of a series of layers of substantially the same thickness λ₀/4. As shown in FIG. 22B, a similar polarizer has two discrete portionsof layers, the first portion having layers of thickness λ₁ /4 (for acharacteristic wavelength λ₁) while the second portion has layers ofthickness λ₂ /4 (for a characteristic wavelength λ₂) to broaden thebandwidth. A variation on this approach is shown in FIG. 22C, in whichthe layers vary gradually in thickness from λ₁ /4 to λ₂ /4, wherein thepolarizer portions gradually vary from a characteristic wavelength λ₁ toa characteristic wavelength λ₂ for broadening the bandwidth.

Additional embodiments of the polarizer of the present invention mayinclude superposing two polarizers 426 of opposite chirality ororthogonality upon one another, as shown as polarizer 470 in FIG. 19A toeffectively comprise a mirror that reflects incident light of apredetermined spectral range, regardless of its polarization. In thisregard, pitches P and P' of the two polarizers are preferably constantand approximately equal to one another, P≈P', for near 100% reflectanceof incident light over a specific spectral band. As shown, an LHCPpolarizer 426 is stacked on an RHCP polarizer 426, which is, in turn,stacked on a substrate 38. The order of stacking is not critical, andboth polarizers 426 are preferably superimposed directly onto oneanother, with no interlayers therebetween. This embodiment may befabricated by first depositing a chiral film on the substrate, thenchanging the rotation of the substrate for depositing the second chiralfilm of opposite chirality. FIG. 19B, shows the pitch distribution ofthe chiral films which is constant. The sign (+ or -) indicates thechirality, which is dependent on the sign of angle β, discussedhereinabove.

In addition, N multiple structures of polarizer 426 may be superposedover one another, in the manner shown in FIG. 19A, each polarizer havinga different constant pitch, P_(j), where j=1, . . . , N. Polarizers 426may be of either single-handed or opposite-handed chiralities. Ifopposite-handed chiralities are utilized, the resulting polarizer willbe a mirror, as discussed hereinabove. Such a stack will generallyprovide a broader polarizing bandwidth, for the similar reason to thatfor a chiral film with gradient pitch. Briefly described, each stack ofP_(j) has a polarizing bandwidth centered at λ_(j). Thus, by suitablespacing for λ₁, λ₂, and λ_(N), a continuous polarizing band from λ₁ toabout λ_(N) can be obtained. In addition, for some applications, thereflection or polarizing band may be predetermined to consist ofdiscrete bands. For example, such a device may be used to reduce and/orremove light at a predetermined spectral range, such as for colorcorrection or alternatively, as a polarizing bandpass filter.

As shown in FIG. 20, a still further embodiment may include a filmpolarizer 472 including stacked polarizers 426' of opposite chiralities,to provide a multiple-pitched, chiral film polarizer. A variation ofpolarizer 472 may include use of single-handed polarizers 426' (notshown). Referring to FIG. 20B, the pitch distribution varies, from, forexample, +1.0P₀ to +1.5P₀, then from -1.0P₀ to -1.5P₀. For normallyincident light, the ordering of pitch gradient is generally notcritical. For example, a pitch distribution from +1.0P₀ to +1.5P₀, thenfrom -1.5P₀ to -1.0P₀, has similar characteristics.

Referring to FIG. 21, a chiral film structure of one or more of thetypes described hereinabove may include a quarter-wave (λ/4) layer forconverting transmitted light from circularly polarized to linearlypolarized, to form a composite linear polarizer. As shown, for example,a polarizer 473 may include a substrate 38 provided with a chiral filmor polarizer 426 disposed thereon, which is in turn, superimposed with,for example, a composite quarter-wave layer 336.

Having described the invention, exemplary methods, such as vacuum vapordeposition methods, used to obtain birefringent film layers forfabrication of the circular and linear polarizers of the presentinvention, are discussed hereinbelow. It has been known since at least1886 that vapor-deposited thin films show birefringence. Moreover,obliquely deposited films are known to exhibit an optical birefringence.There is extensive literature regarding metallic films for magneticrecording and SiO_(x) (x=1, 2) films for liquid crystal alignment.Slocum has exploited oblique evaporation techniques to depositanisotropic silver films on glass substrates to create infrared dichroiclinear polarizers, as disclosed in the above-reference '907 patent,which is hereby incorporated by reference in its entirety, herein. It isalso well known to those skilled in the art that plates deposited at anoblique incidence angle have a columnar structure with the columnsoriented according to the plane of incidence. Even though the filmmaterial may itself by isotropic, the obliquely evaporated film isbirefringent because of the anisotropic nature of the film morphology.That is, on a microscopic level, obliquely evaporated films are seen tobe comprised of closely packed needles, or micro-ellipsoids. Thus thedeposited film displays optical anisotropy between directions paralleland perpendicular to the long axes of the micro-needles.

With reference now to FIG. 12A, column or optic axis inclination angleα, of an obliquely vapor deposited film layer 538 is related to vapordeposition angle θ of evaporant vapor stream 3 by an empirical formula,

    α=kθ,                                          (4)

where k is a material dependent constant. Angle α is complementary toangle γ, (α=90°-γ) where γ is defined hereinabove, in connection withFIG. 8. For many deposited materials, this relationship may be describedby the formula:

    tanα=1/2tanθ                                   (4a)

Angle α may be any angle within the range of from 0° up to approximately89°.

In theory, films with α=0° have the largest birefringence, while filmswith an angle α of 90° having no birefringence. However, experimentalresults (FIG. 12B) have shown that the largest birefringence (Δn) isgenerally obtained by depositing film at an angle of approximately 65°to obtain an angle α of approximately 45°.

Such films are optically biaxial; however, for normally incident light(light incident along the z-axis as shown), the film appears uniaxial.For example, as shown, optic axis c is in the xz-plane with aninclination angle α relative to the xy-plane. Since x-polarized lighthas an electric field oscillating in the ±x-axis direction, the lightexperiences no difference with respect to whether angle α is positive ornegative (+α or -α). Likewise y-polarized light has an electric fieldoscillating in the ±y-axis direction, the light experiences nodifference with respect to whether angle α is positive or negative (+αor -α). Film 538 thus has an average refractive index n_(av) andbirefringence Δn. By varying the deposition angle θ, the birefringenceΔn can be engineered.

Referring now to FIG. 12B, experimental results are shown forbirefringence Δn vs deposition angle θ for SiO films. The birefringenceΔn increases gradually with θ until it attains a maximum value of 0.012at about 65° and decreases rapidly. As expected, films deposited atnormal incidence do not exhibit any optical birefringence. Otherobliquely deposited oxide films have been shown to exhibit a similartrend.

Suitable materials for oblique vapor deposition of the birefringent thinfilms of the present invention include lithium (Li), barium (Ba),yttrium (Y), titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V),niobium (Nb), tantalum (Ta), molybdenum (Mo), tungsten (W), rhenium(Re), boron (B), silicon (Si), germanium (Ge), tin (Sn), lead (Pb),antimony (Sb), cerium (Ce) and bismuth (Bi) oxides. Metal fluorides suchas MgF₂ and sulfides such as ZnS may also act as birefringent filmsusing oblique deposition. It is expected that other materials, includingorganic compounds, may be obliquely deposited to form suitablebirefringent films for use in the present invention.

Chiral film or a circular polarizer of the type described hereinabovewith respect to polarizer 426, is obtained by progressively rotating thesubstrate in discrete steps during the deposition process, resulting ina chiral film that is a stack of multiple birefringent thin layers. Asthe deposition accumulates, the optic axis traces a helical patternextending in the direction normal to the film (z-axis). The helicalpitch P is a function of the vapor deposition rate relative to therotation rate. The `handedness` or chirality is set by the direction ofthe substrate rotation. The present invention thus includes rotation ofthe substrate is rotated in discrete steps during the depositionprocess, rather than continuously, resulting in a chiral film that is astack of multiple birefringent thin layers. The advantage of the presentdeposition technique is that it allows sufficient time to form thebirefringence-inducing columnar structure on a stationary substrate foreach birefringent film layer.

Referring now to FIG. 13, means for oblique deposition layers of thepresent invention is shown. A vacuum thin-film deposition system 560suitable for oblique deposition, is a variation of a conventional vacuumvapor deposition system. A vacuum chamber 562, whose pressure ismaintained by a vacuum pump system (not shown) is connected at outletorifice 564. A conventional pump system including an oil-diffusion pumpand a mechanical pump can maintain a pressure of 10⁻⁶ Torr in chamber562. Other pump systems such as a molecular turbo pump or cryogenicpump, may also be used. Advantages and disadvantages of various vacuumpumps are well-known to those skilled in the art. An evaporant vaporstream source 566 is disposed inside the vacuum chamber, and a series ofbaffles 568 are provided to restrict divergence of the evaporant vaporbeam 570. Several vapor sources may be employed. Typically, vapor streamsource 566 contains an evaporant material to be deposited. The evaporantmaterials is heated by various means, such as electrical current,electron beam, pulsed laser irradiation and others to achieve apredetermined vapor pressure. Thermal evaporation, E-beam deposition andother techniques are well-known to those skilled in the art. Amechanical shutter 572 is preferably provided to gate vapor beam 570.Gating is achieved by mechanically blocking evaporant vapor beam 570 toprevent the beam from reaching substrate 574. The substrate is situatedon a substrate holder 576. The substrate holder is pivotably attached bya coupling 578 to a post 580, to enable the substrate to be tilted at adesired angle θ with respect to the evaporant vapor stream. Thesubstrate is preferably heated radiantly by a suitable lamp 582 (i.e. atungsten-halogen lamp) to control substrate temperature. Alternatively,resistive heating can also be employed. A film thickness monitor 584 maybe used to monitor the deposition rate. As shown, a stepper motor 586 ispreferably employed to rotate the substrate with respect to theevaporant vapor stream for depositing film for a circular polarizer asdescribed hereinabove.

This system may also be utilized for construction of a linear polarizeras described hereinabove. In this regard, substrate 574 will be tiltedto ±θ with respect the vapor beam. For isotropic layers, the substrateis tilted and rotated rapidly to prevent columnar growth.

Referring now to FIG. 14, a vapor deposition system 660 suitable forfabrication of linear polarizer 126 (FIG. 3A) at a high throughput, isshown. System 660 has six deposition chambers, each having a substrate(2E, 3E, 4E, 5E, 6E, and 7E, respectively) and vapor source. Forexample, the substrate 2E is deposited for a thickness t₁ by vapor beam2B emitted by the vapor source 2A of evaporant material A at an obliqueangle of θ with normal to the substrate. For simplicity of theillustration, baffles, vapor beam shutter and others for the evaporantvapor beam 2B are not shown. After the desired film thickness isdeposited, the substrate carrier 662 is rotated clockwise by 60°, so thesubstrate is deposited for a thickness t₂ by a vapor beam 3B emitted bya vapor source 3A, of material A at an oblique angle θ'=-θ. According tothe technical teaching hereinabove, a birefringent layer of totalthickness t_(A) =t₁ +t₂, that consists of two sublayers is obtained. Asdiscussed hereinabove, the sublayer structure reduces problemsassociated with growth in columnar diameter. After the birefringentlayer is deposited, the substrate carrier is rotated clockwise by 60°again, so the substrate is deposited for a thickness t_(B) by a vaporbeam 4B emitted by a vapor source 4A at normal incidence to thesubstrate. The film thus deposited is optically isotropic. As thesubstrate carrier sequentially rotates clockwise by 60°, substrates 5E,6E and 7E are similarly deposited with material A by vapor beams 5B, 6Band 7B emitted from vapor sources 5A, 6A and 7A respectively. Amultilayer coating of periodic structure, consisting alternating ofbirefringent and isotropic thin layers, thus deposited on the substrate.Each birefringent layer consists of two sublayers deposited at +θ and -θwith respect to the normal to the film stack. It is clear that sixsubstrates can be deposited simultaneously to achieve high throughput.

Another vapor deposition system 760 suitable for continuous depositionof flexible substrate is shown schematically in FIG. 15. The vapordeposition system is suitable for high throughput. A cylindrical drum762 with a relatively large diameter holds a flexible substrate 764.Polyester, Mylar, and/or polycarbonate thin films are suitable as asubstrate. As the drum rotates counterclockwise, the substrate movesunder various vapor sources, and a particular film layer is deposited,to result in a birefringent or isotropic layer, depending on theincidence angle of the evaporant vapor stream, normal to the substrate.For example, the substrate under vapor source 4A is deposited for athickness t₁ with evaporant material A at an oblique angle of θ withnormal to the substrate. As that portion of the substrate is moved undervapor source 5A, a thickness t₂ of the evaporant material A is depositedat an oblique angle of θ'=-θ with normal to the substrate. Bafflessimilar to baffles 568 discussed hereinabove with respect to FIG. 13,are shown at 4C, 4D, 5C, 5D, 6C and 6D. According to the technicalteaching hereinabove, a birefringent layer consisting of two sublayersis obtained. This layer has a total thickness t_(A) =t₁ +t₂. As thatportion of the substrate is moved under vapor source 6A, a thicknesst_(B) of the evaporant material B is deposited at normal incidence tothe substrate. The film thus deposited is optically isotropic. As thesubstrate carrier rotates continuously, a multilayer coating of periodicstructure, consisting alternating of birefringent and isotropic thinlayers, are deposited on the substrate.

To utilize system 760 to deposit chiral films, such as, for example,polarizer 426 (FIG. 8A), the vapor sources are oblique at θ with thesubstrate normal. As the substrate is advanced from one source to next,the substrate is rotated by angle β in the substrate plane to form afilm stack according to the present invention.

A vapor deposition system 860 suitable for continuous deposition onflexible substrate is shown in FIG. 16. This vapor deposition system isalso suitable for high throughput. A long flexible substrate 862 isattached on a conveyor comprising drum 864, drum 866 and belt 868.Polyester, myler, polycarbonate thin films are suitable as a substrate.As the substrate moves counterclockwise under various vapor sources, anda particular film layer is deposited, to result a birefringent orisotropic layer. For example, the substrate under vapor source 5A isdeposited for a thickness t₁ with evaporant material A at an obliqueangle of θ with normal to the substrate. As that portion of thesubstrate is transported under vapor source 6A, a thickness t₂ of theevaporant material A is deposited at an oblique of θ'=-θ with normal tothe substrate. Baffles similar to baffles 568 discussed hereinabove withrespect to FIG. 13, may be utilized as shown, for example at 7C, 7D, 8C,8D, 10C and 10D. According to the technical teaching above, abirefringent layer consisting of two sublayers is obtained. This layerhas a total thickness t_(A) =t₁ +t₂. At that portion of the substrate istransported under vapor source 7A, a thickness t_(B) of the evaporantmaterial B is deposited at normal incidence to the substrate. The filmthus deposited is optically isotropic. As the substrate carrier rotatescontinuously, the substrate is similarly deposited with material byvapor beams 8B, 9B and 10B emitted from vapor sources 8A, 9A and 10A,respectively. A multilayer coating of periodic structure, consistingalternating of birefringent and isotropic thin layers, is thus depositedon the substrate.

Flexible substrates such as polyester, Mylar, polycarbonate thin filmstend to exhibit residual birefringence, which has depolarizing effects,after light transmits through the polarizing film stack. Since thelinearly polarizing film structures of the present invention operate byreflection, these polarizers may be utilized in a reflective modewherein such small residual birefringence in the substrate hasnegligible effect.

As shown in FIG. 17A, a polarizing device 960 according to the presentinvention is utilized in reflective mode, i.e., reflected light is thelight of interest. Device 960 is a multilayer thin-film structure,consisting of a stack of alternating birefringent and isotropic layers92 to 97 deposited on a flexible substrate 91 which may have residualbirefringence. Device 960 is placed so that the uppermost layer 97 facesincoming light 98. By virtue of polarization-selective reflectance,linearly polarized light 99 within the reflectance band of the device isreflected, while transmitted light 100 is partially depolarized becauseof the birefringence of the substrate. Since the polarizer is operatedin its reflectance mode, the reflected light does not interact with thesubstrate at all, so it does not affect the polarization state of thereflected light. Similar effects pertain to circular polarizers disposedon a birefringent substrate.

Conversely, FIG. 17B shows the usage for the transmissive mode, i.e.,transmitted light is of interest. In this configuration, device 960' isoriented with substrate 91 facing the incoming light 98. By virtue ofpolarization-selective reflectance, linearly polarized light 99 istransmitted, if the incident light has a spectral bandwidth within thereflective band of the polarizing device. Substrate 91 may exhibitbirefringence. Thus, in its transmissive mode, the light is firsttransmitted through the substrate, where its polarization state maybecome `perturbed.` The light then passes through the multilayerpolarizing film stack where it is polarized linearly or circularly. Thisis preferred arrangement. On the other hand, if the light were passedthrough the film stack first and then through the substrate, thetransmitted light emerging therefrom may not be `purely` polarizedlinearly of circularly.

The embodiments of the present invention as described hereinabove thusserve to provide a non-absorbing, normal-incidence polarizer by means ofnominally completely reflecting light polarized in one orientation ofpolarization. Advantageously, by reflecting, rather than absorbing thiscomponent of incident light, a polarizer of the present invention isable to withstand greater light intensities than prior art polarizers,without excessive heating. An additional benefit of utilizing reflectionrather than absorption, is that the present invention may be fabricatedfrom high temperature resistant inorganic materials, rather thanoptically absorptive organic materials. This aspect further increasesthe energy handling capability of the present invention, by enabling thepolarizer to function at temperatures as high as approximately 200degrees C or more.

The foregoing description is intended primarily for purposes ofillustration. Although the invention has been shown and described withrespect to an exemplary embodiment thereof, it should be understood bythose skilled in the art that the foregoing and various other changes,omissions, and additions in the form and detail thereof may be madetherein without departing from the spirit and scope of the invention.

Having thus described the invention, what is claimed is:
 1. A lightpolarizer comprising:a substrate; a plurality of discrete layers ofbirefringent film disposed on said substrate in superposed relation toone another; each of said plurality of discrete layers beingsubstantially planar and defining a reference plane; each one of saidplurality of discrete layers having an optic axis extending obliquelyrelative to said reference plane, said optic axis having a planarcomponent thereof extending parallel to said reference plane; saidplanar component of at least one of said plurality of discrete layersbeing angularly offset from said planar component of an other of saidplurality of discrete layers.
 2. The light polarizer as set forth inclaim 1, wherein said planar component of each one of said plurality ofdiscrete layers is angularly offset from each adjacent one of saidplurality of discrete layers.
 3. The light polarizer as set forth inclaim 2, wherein said planar component of each successive one of saidplurality of discrete layers is angularly offset relative each precedingone of said plurality of discrete layers, by an oblique, acute angle inone of a positive and negative direction to provide said light polarizerwith a chirality being one of right-handed and left-handed forcircularly polarizing light incident thereon.
 4. The light polarizer asset forth in claim 3, wherein said oblique, acute angle is constant foreach successive one of said plurality of discrete layers.
 5. The lightpolarizer as set forth in claim 3, further comprising a pitch P₀ =λ₀/n_(av), where λ₀ is a characteristic wavelength of said light polarizerand n_(av) is an average index of refraction of said light polarizer. 6.The light polarizer as set forth in claim 5, wherein said pitchcomprises a predetermined number J of said plurality of discrete layers,of substantially equal thickness t.
 7. The light polarizer as set forthin claim 6, wherein said thickness t=P₀ /J and said predetermined numberJ is an integer.
 8. The light polarizer as set forth in claim 6, whereinsaid predetermined number J is a non-integer.
 9. The light polarizer asset forth in claim 3, wherein said plurality of discrete layers furthercomprises a plurality of portions indexable as j=1, 2, . . . , N,disposed in superposed orientation with one another;each of saidplurality of portions including a predetermined number of the pluralityof discrete layers, and having a chirality being one of right-handed andleft-handed; each of said plurality of portions having a pitch P_(j)=λ_(j) /n_(avj), where λ_(j) is a characteristic wavelength, and n_(avj)is the average index of refraction, respectively, of each of theplurality of portions j.
 10. The light polarizer as set forth in claim9, wherein said plurality of portions further comprises a first portionj=1 and a second portion j=2, said chirality of said first portion beingopposite to said chirality of said second portion.
 11. The lightpolarizer as set forth in claim 10, wherein:said first portion has afirst pitch P₁ and said second portion has a second pitch P₂, said firstpitch P₁ and said second pitch P₂ being substantially constantthroughout said first portion and said second portion, respectively; andsaid first pitch P₁ and said second pitch P₂ being substantially equalto one another, wherein said light polarizer reflects substantially 100%of incident light having a predetermined spectral band.
 12. The lightpolarizer as set forth in claim 9, wherein said plurality of portionsfurther comprises a first portion j=1 and a second portion j=2, saidchirality of said first portion being substantially identical to saidchirality of said second portion.
 13. The light polarizer as set forthin claim 9, wherein:said pitch of each one of said plurality of portionsis constant therethrough; at least one of said plurality of portionshaving a pitch distinct from that of at least one other of saidplurality of portions; said plurality of portions all being of identicalchirality.
 14. The light polarizer as set forth in claim 9, wherein saidpitch of each of said plurality of portions is constant therethrough;atleast one of said plurality of portions having a pitch distinct from atleast one other of said plurality of portions; said plurality ofportions including portions having opposite chiralities to one another.15. The light polarizer as set forth in claim 3, further comprising apitch, said pitch being variable along a direction orthogonal to saidreference plane.
 16. The light polarizer as set forth in claim 15,wherein said pitch is varied by selectively varying said oblique, acuteangle of successive ones of said plurality of discrete layers.
 17. Thelight polarizer as set forth in claim 16, wherein said pitch is variedby providing selected ones of said discrete layers with dissimilarthickness t.
 18. The light polarizer as set forth in claim 15, whereinsaid pitch is varied by providing selected ones of said discrete layerswith dissimilar thicknesses t.
 19. The light polarizer as set forth inclaim 15, wherein said plurality of discrete layers further comprise aplurality of portions disposed in superposed orientation with oneanother;each of said plurality of portions including a predeterminednumber of said plurality of discrete layers, and having a chiralitybeing one of right-handed and left-handed.
 20. The light polarizer asset forth in claim 19, wherein said plurality of portions furthercomprise a first portion and a second portion, said chirality of saidfirst portion being opposite to said chirality of said second portion.21. The light polarizer as set forth in claim 19, wherein said pluralityof portions are all of the same chirality.
 22. The light polarizer asset forth in claim 3, further comprising a quarter-wave layer ofbirefringent film superimposed thereon, said quarter-wave layer beingsubstantially planar and having a thickness t=λ₀ /4, where λ₀ is acharacteristic wavelength of said light polarizer, wherein saidquarter-wave layer converts light incident thereon from circularlypolarized to linearly polarized.
 23. The light polarizer as set forth inclaim 2, wherein said planar component of each successive one of saidplurality of layers is orthogonally offset relative each preceding oneof said plurality of layers.
 24. The light polarizer as set forth inclaim 23, wherein said plurality of discrete layers further comprises atleast two portions indexable as j=1, 2, . . . N, disposed in superposedorientation with one another;each of said at least two portionsincluding a predetermined number of said plurality of discrete layers;each of said at least two portions having a characteristic wavelengthλ_(j) ; and at least two of said characteristic wavelength λ₁ and λ₂being distinct from one another wherein polarizing bandwidth of saidlight polarizer is relatively increased.
 25. The light polarizer as setforth in claim 24, wherein each one of said predetermined number of saidplurality of discrete layers further comprises a quarter-wave layer,having a thickness t=λ_(j) /4.
 26. The light polarizer as set forth inclaim 25, wherein said plurality of discrete layers further comprises atleast one intermediate layer disposed between said at least twoportions, said at least one intermediate layer having a thicknessbetween λ₁ /4 and λ₂ /4 so that said light polarizer varies graduallyfrom said characteristic wavelength λ₁ to said characteristic wavelengthλ₂.
 27. The light polarizer as set forth in claim 23, wherein each oneof said plurality of layers further comprises a quarter-wave layer,having a thickness t=λ₀ /4, where λ₀ is a characteristic wavelength ofsaid light polarizer.
 28. The light polarizer as set forth in claim 23,wherein at least one of said quarter-wave layers further comprises acomposite layer having a plurality of discrete superimposedsublayers;each one of said plurality of discrete superimposed sublayersbeing substantially planar and extending in said reference plane; eachone of said plurality of superimposed sublayers having a sublayer opticaxis and a sublayer planar component thereof; said sublayer planarcomponents of each one of said plurality of superimposed sublayersextending in said reference plane parallel to one other, to define saidplanar component of said composite layer.
 29. The light polarizer as setforth in claim 28, further comprising a plurality of said compositelayers.
 30. The light polarizer as set forth in claim 29, wherein eachone of said plurality of discrete layers comprise one of said compositelayers.
 31. The light polarizer as set forth in claim 1, wherein saidsubstrate is transparent in a desired wavelength region of light. 32.The light polarizer as set forth in claim 1, wherein each one of saidplurality of discrete layers are fabricated from an inorganic material.33. The light polarizer of claim 32, wherein each one of said pluralityof discrete layers is fabricated from a material selected from the groupconsisting of lithium (Li), barium (Ba), yttrium (Y), titanium (Ti),zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta),molybdenum (Mo), tungsten (W), rhenium (Re), boron (B), silicon (Si),germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), cerium (Ce), bismuth(Bi) oxides, lithium fluoride (LiF), calcium fluoride (CaF₂), magnesiumfluoride (MgF₂) and zinc sulfide (ZnS).
 34. The light polarizer as setforth in claim 1, wherein at least one of said plurality of discretelayers is fabricated by depositing material at an oblique depositionangle.
 35. A light polarizer comprising:a substrate; a plurality ofdiscrete layers of birefringent film disposed on said substrate insuperposed relation to one another; each of said plurality of discretelayers being substantially planar and defining a reference plane; eachone of said plurality of discrete layers having an optic axis, saidoptic axis having a planar component thereof extending parallel to saidreference plane; said planar component of each successive one of saidplurality of discrete layers being angularly offset relative to saidplanar component of said preceding one of said plurality of discretelayers by an oblique, acute angle in one of a positive and negativedirection to provide said light polarizer with a chirality being one ofright-handed and left-handed for circularly polarizing light incidentthereon; and having a pitch P₀ =λ₀ /n_(av), where λ₀ is a characteristicwavelength of said light polarizer and n_(av) is an average index ofrefraction of said light polarizer.